The invention relates to making fiber sheets, and more particularly multiaxial sheets formed by superposing and linking together a plurality of unidirectional fiber sheets disposed in different directions.
A field of application of the invention lies in making multiaxial fiber sheets for forming reinforcing plies for preparing composite material parts. The intended materials are particularly those constituted by fiber reinforcement which can be organic or inorganic, or precursors therefor such as fibers of polymer, glass, carbon, ceramic, para-aramid, . . . , which reinforcement is densified by an organic matrix, e.g. a resin, or an inorganic matrix, e.g. glass, carbon, or ceramic.
It has been known for a long time to make multiaxial fiber sheets by superposing unidirectional sheets, i.e. made up of threads or fibers that are oriented essentially in a single direction, the unidirectional sheets being superposed in different directions.
A common technique consists initially in making the unidirectional fiber sheets, and in giving them sufficient cohesion to enable them to be handled without dispersing the elements making them up.
A commonly proposed solution consists in bonding together the elements forming the warp of the unidirectional sheets by threads extending in the weft direction. This inevitably results in undulations being formed which, when a plurality of sheets are superposed and pressed against one another, can cause fibers to be crushed and broken, thereby creating discontinuities. That degrades the multiaxial sheets made in that way and consequently degrades the mechanical properties of the composite material parts prepared from such multiaxial sheets.
To remedy that drawback, a well-known solution consists in using bonding threads of number and weight that are as small as possible. Document GB-A-1 190 214 (Rolls Royce Limited) concerning sheets of carbon precursor fibers, and document FR-A-1 469 065 (Les Fils d""Auguste Chomarat and Cie), concerning sheets of glass fibers, illustrate that approach. Nevertheless, it is clear that the above-mentioned drawback is diminished but not eliminated.
It is also proposed in document EP-A-0 193 478 (Etablissements Les Fils d""Auguste Chomarat and Cie) to use bonding fibers but made of a heat-fusible material. During the preparation of the composite material, the temperatures used can cause the bonding threads to melt at least in part, thereby reducing the extra thickness where they cross the warp elements. However it is necessary for the material of the bonding fibers to be compatible with the nature of the matrix of the composite material, which greatly limits the use of that method.
Another solution mentioned in document FR-A-1 394 271 (Les Fils d""Auguste Chomarat and Cie) consists in placing glass fiber threads parallel to one another and in bonding them together chemically, the binder used being soluble in the matrix. In that case also, the need for compatibility between the binder and the matrix limits applications of the method. Furthermore, no means is described to enable the threads to be placed parallel to one another, and it will readily be understood that making wide sheets on an industrial scale gives rise to real practical difficulties. Finally, the resulting sheet is not free from undulations resulting from the threads being placed side by side.
Yet another solution consists in spreading out a plurality of tow, bringing together the resulting unidirectional fiber strips in a side by side configuration to form a sheet, and in imparting transverse cohesion to the sheet by needling. Such a method is described in particular in document U.S. Pat. No. 5,184,387 (assigned to Aerospace Preforms Limited) where the tows used are made of carbon precursor fibers capable of being needled without being broken. Nevertheless, multiaxial sheets are not made by superposing those unidirectional sheets. According to that document, annular sectors are cut out from the unidirectional sheet to form annular plies which are superposed and needled.
To avoid the need to give even temporary cohesion to unidirectional sheets for making multiaxial sheets, it is known to make the multiaxial sheets directly by forming a plurality of unidirectional sheets and by superposing them in different directions without any intermediate handling. The superposed sheets can be connected to one another by bonding, by sewing, or by knitting.
Documents illustrating that technique are, for example, documents: U.S. Pat. Nos. 4,518,640, 4,484,459, and 4,677,831.
In document U.S. Pat. No. 4,518,640 (assigned to Karl Mayer) reinforcing threads are introduced into the sheet while it is being formed, thereby making it possible to provide bonding without piercing through the fibers. Nevertheless, that gives rise to openings being present in the multiaxial sheet, which openings produce surface discontinuities.
In document U.S. Pat. No. 4,484,459 (assigned to Kyntex Preform), each unidirectional sheet is formed by causing a thread to pass around spikes carried by two parallel endless chains, such that the portions of the threads that extend freely between the spikes are mutually parallel. Unidirectional sheets are formed by guiding the respective threads in different directions, and they are bonded to one another by sewing. With that technique it is not possible to have reinforcing threads in the longitudinal direction of the multiaxial sheet; unfortunately, it is often necessary to place reinforcing elements in that main direction. In addition, if a large amount of tension is exerted on the threads to guarantee parallelism in each sheet, then the portions of the threads extending between the spiked chains can tend to become rounded by the fibers tightening, thereby giving rise to openings in the multiaxial sheet. Finally, it will be observed that that technique does not make a very high production speed possible given the time required for forming each unidirectional sheet.
In document U.S. Pat. No. 4,677,831 (assigned to Liba Maschinenfabrik GmbH), the technique described consists in displacing a main unidirectional sheet longitudinally parallel to the direction of the elements which make it up, and in laying transverse unidirectional sheets thereon in directions that make predetermined angles with the direction of the main sheet (0xc2x0), for example +45xc2x0 and xe2x88x9245xc2x0 and/or +60xc2x0 and xe2x88x9260xc2x0. The transverse sheets are laid by a laying process between two spiked chains situated on either side of the main sheet. That technique which does not necessarily require a main sheet to be present, also suffers from several drawbacks.
Thus, it is necessary to eliminate the marginal zones where the transverse sheets turn around the spikes. Unfortunately, the wider the transverse sheets, the larger the marginal zones, and the larger the losses of material due to their being eliminated, and it is also more difficult to turn the sheets on the spikes. This greatly limits the width that can be used for the transverse sheets. In addition, the above-mentioned drawback of possible irregularity in the multiaxial sheet is also to be found, in particular due to the formation of holes because of the tensions that it is necessary to apply to the elements of the transverse sheets in order to hold them parallel during laying.
In addition, relatively high stitch density is necessary immediately after laying in order to confer sufficient strength to the resulting multiaxial sheet. In addition to making it impossible to preserve a smooth surface state, this high stitch density affects the flexibility of the multiaxial sheet and limits its deformability in use, e.g. by draping.
Furthermore, when a main sheet (0xc2x0) is provided, it is necessary to support it while the transverse sheets are being laid, such that all of them are to be found on the same side of the main sheet. Reinforcing elements are indeed provided that extend in the main direction (0xc2x0), but the resulting multiaxial sheet is not symmetrical between its faces. Unfortunately, such symmetry is advantageous to facilitate the construction of regular reinforcement and it is therefore desirable to place the main direction at 0xc2x0 in the middle of the multiaxial sheet, between its faces.
It should also be observed that a drawback common to those techniques using threads for forming unidirectional sheets lies in obtaining multiaxial sheets which firstly present surface roughness due to the threads, and secondly cannot be as thin as it is sometimes desired.
Finally, a method of making a multiaxial sheet from unidirectional sheets is also described in document GB-A-1 447 030 (Hyfil Limited). A first unidirectional sheet of warp-forming carbon fibers is pre-needled and another, weft-forming unidirectional sheet is bonded to the first, likewise by needling. The pre-needling of the first sheet seeks to displace fibers from the side where the second sheet is to be placed, in order to contribute to bonding therewith. It will be observed that the unidirectional sheets used are made coherent by a bonding thread, as described in above-mentioned document GB-A-1 190 214, with the drawbacks that result therefrom.
It should also be observed that the above-mentioned known techniques all suffer from a drawback which lies in the relatively high cost of multiaxial fiber sheets when they are made using carbon fibers. There exists a need to reduce the cost of such sheets, in particular so as to extend their field of application.
An object of the invention is to propose a novel method of making multiaxial fiber sheets, in particular to enable the cost of making such sheets to be reduced, so as to cause multiaxial sheets made with fibers that have the reputation of being expensive, such as carbon fibers, to be more attractive.
Another object of the invention is to propose a method enabling xe2x80x9cmirrorxe2x80x9d multiaxial sheets to be made, i.e. multiaxial sheets presenting symmetry relative to a midplane, in particular relative to a main unidirectional sheet (0xc2x0), which sheet is therefore situated between transverse unidirectional sheets making opposite angles relative to the main direction.
Another object of the invention is to propose a method enabling multiaxial fiber sheets to be made that present a surface of smooth appearance without irregularities such as holes or roughnesses.
Another object of the invention is to propose a method enabling multiaxial fiber sheets to be made requiring only a very low density of bonding transversely to the unidirectional sheets making them up in order to ensure coherence, thereby enabling good deformability of the multiaxial sheets to be preserved.
Another object of the invention is to provide multiaxial fiber sheets having the above properties while also being of great length, and of small thickness and weight (per unit area).
Another object of the invention is to propose a laying method and machine enabling multiaxial fiber sheets to be made from unidirectional sheets that can be relatively wide, while conserving good surface regularity and limiting losses of material.
In one aspect, the invention provides a method of making a multiaxial fiber sheet, the method comprising the steps consisting in superposing a plurality of unidirectional sheets in different directions, and in bonding the superposed sheets together, in which method, to make at least one unidirectional sheet, at least one tow is spread so as to obtain a sheet of substantially uniform thickness, having a width of not less than 5 cm and a weight of not more than 300 grams per square meter (g/m2), and cohesion is imparted to the unidirectional sheet enabling it to be handled prior to being superposed with at least one other unidirectional sheet.
In a feature of the method, to make at least one of the unidirectional sheets, a plurality of tows are used, the tows are spread so as to form unidirectional strips, and the strips are placed side by side so as to obtain a unidirectional sheet having a width of not less than 5 cm and weighing not more than 300 g/m2.
To further improve an advantage of the method, in particular when using carbon, at least one of the unidirectional sheets is preferably obtained by spreading at least one tow having a number of filaments equal to or greater than 12 K (12,000 filaments) and possibly as many as 480 K (480,000 filaments) or more.
A similar technique can be used with all technical fibers.
An advantage of the method is thus to use large tows, in particular the largest tows available for various kinds of fiber.
For given weight, particularly with carbon, the cost of a fat tow is much less than that of a thin tow or thread of the kind which, so far as the Applicants are aware, are those used in the state of the art for making multiaxial sheets.
By way of illustration, the following table applies to commercially available carbon threads or tows formed using different numbers of filaments, and gives the weights that can be obtained for a unidirectional sheet, depending on whether it is formed by mutually parallel threads as in the prior art, or by spreading tows as in the present invention. The threads or tows are made of high strength or high modulus carbon with a polyacrylonitrile or an anisotropic pitch precursor.
A tow is spread or a plurality of tows are spread and juxtaposed, so as to form at least one unidirectional sheet having weight per unit area no greater than 300 grams per square meter (g/m2), thus making it possible from a limited number of heavy tows to provide a sheet of relatively broad width, i.e. at least 5 cm, and preferably at least 10 cm.
The use of unidirectional sheets of relatively light weight makes it possible to conserve this property in multiaxial sheets made up of such unidirectional sheets.
In addition, contrary to the above-mentioned prior art techniques using sheets of parallel threads, spreading tows until lightweight sheets are obtained causes multiaxial sheets to be made that do not have surface defects such as holes or undulations, and that have smooth surface appearance. It is also possible with the method of the invention to use fibers that are fragile.
When the unidirectional sheet is built up from discontinuous filaments, cohesion can be imparted thereto by matting the filaments to a small extent. To this end, the sheet can be subjected to needling or it can be exposed to a jet of water under pressure, the sheet being disposed over a plate. The sheet can then be widened without losing its cohesion.
In all cases, regardless of whether the unidirectional sheet is made of filaments that are continuous or discontinuous, cohesion can be imparted thereto by providing a chemical bonding agent which may optionally be suitable for being eliminated (or sacrificed). The agent is advantageously applied by spraying a liquid compound onto the sheet or by passing it through a bath. Cohesion can also be provided by dusting a heat-fusible or thermo-adhesive polymer in powder form onto the sheet.
It is also possible to envisage imparting transverse cohesion to at least one of the unidirectional sheets used by fixing by means of at least one heat-fusible or thermo-adhesive film or thread, or indeed by forming a line of adhesive, e.g. an adhesive in solution in an evaporatable solvent.
The method of the invention seeks more particularly to make a continuous multiaxial sheet having a longitudinal direction, by fetching at least one transverse unidirectional sheet onto a moving support that moves in a direction of advance parallel to the longitudinal direction of the multiaxial sheet, the or each transverse unidirectional sheet being fetched as successive segments that are adjacent or that overlap in part and that form the same selected angle relative to the direction of advance.
The cohesion of the superposed unidirectional sheets makes it possible to make multiaxial sheets without constraints on laying the unidirectional sheets relative to one another, thus providing great flexibility concerning the order in which the unidirectional sheets are superposed. It is thus possible to make multiaxial sheets that present symmetry relative to a midplane (xe2x80x9cmirrorxe2x80x9d symmetry), in particular relative to a longitudinal middle unidirectional sheet whose direction is parallel to the direction of advance, together with at least two transverse unidirectional sheets disposed on either side of the longitudinal sheet and forming opposite angles relative thereto.
In a preferred implementation of the method, each of the successive segments forming a transverse sheet is fetched by moving the sheet over a length substantially equal to the dimension of the multiaxial sheet as measured parallel to the direction of the transverse sheet, by cutting off the segment fetched in this way, and by depositing the cutoff segment on the moving support or the multiaxial sheet that is being made. Advantageously, the transverse sheet is reinforced in the zones where it is cut, e.g. by fixing a film on at least one of its faces.
It will be observed that laying transverse sheets in successive cutout segments makes it possible to limit losses of material compared with the known technique of laying by turning the sheet around spikes. In addition, working in this way avoids damaging the fibers, and therefore makes it possible to lay fibers that are fragile, such as high modulus carbon fibers or carbon fibers based on anisotropic pitch, or ceramic fibers. In addition, restarting the laying process after a break in transverse sheet feed is made much easier compared with the case where the transverse sheets are formed by a set of parallel fibers that are not bonded together.
In another aspect, the invention provides a unidirectional or multiaxial fiber sheet as obtained by the above method.
In yet another aspect, the invention provides making composite material parts that comprise fiber reinforcement densified by a matrix, in which parts the fiber reinforcement is made from at least one such unidirectional or multiaxial sheet.
In a further aspect, the invention provides a laying machine enabling the preferred implementation of the method to be performed.
To this end, the invention provides a laying machine for making a multiaxial fiber sheet by superposing unidirectional fiber sheets in different directions, the machine comprising:
apparatus for advancing the multiaxial sheet, the apparatus comprising support means for supporting the multiaxial sheet that is being made and drive means for driving the support means in a direction of advance;
feed means for feeding longitudinal unidirectional sheet in a direction parallel to the direction of advance;
a plurality of cross-laying devices each including feed means for feeding the cross-laying device with continuous unidirectional sheet, a moving grasping head for taking hold of the free end of a sheet, and means for laying successive segments of sheet parallel to a transverse direction at a selected angle relative to the direction of advance, said laying means comprising means for driving the grasping head; and
bonding means for bonding the superposed unidirectional sheets together, the bonding means being located downstream from the support means in the direction of advance,
in which machine:
each cross-laying device includes cutter means; and means are provided for performing successive cycles comprising, for each cross-laying device, grasping the free end of a unidirectional sheet by means of the grasping head, moving the grasping head to fetch a segment of unidirectional sheet, cutting off the fetched segment of unidirectional sheet, and laying the cutoff segment of unidirectional sheet on the support means.
An Important advantage of such a machine lies in the possibility of laying unidirectional sheets of relatively broad width, including in the transverse directions.
Superposed unidirectional sheets can be bonded together in various ways, e.g. by sewing, by knitting, by needling, or by adhesive, e.g. by spraying an adhesive agent or by inserting a heat-fusible or thermo-adhesive film or thread between the sheets. A bonding agent that may possibly have been used for providing cohesion within unidirectional sheets can be reactivated to bond the sheets to one another.